Laser Powder Bed Fusion of gamma-Titanium Aluminide Ti48-2-2: Manufacturing and Testing of Highly-Loaded Compressor Wheels

Abstract

The laser-based metal powder bed fusion process (PBF-LB/M) offers significant technological advantages in the aerospace sector for the production of complex and integrated parts, e.g. internally-cooled double-wall turbine blades, regeneratively-cooled rocket engines or compressor wheels that are commonly difficult to manufacture. Especially for aeronautic gas turbine or compressor applications, high-temperature materials with high-specific strength such as titanium aluminides (TiAl) are of major interest. During the last years we investigated the PBF-LB/M processing, heat treatments and phase transformations of different titanium aluminides including the -stabilized TNM-B1 (Ti-43.5Al-4Nb-1.0Mo-0.1B) and TNB-V4 (Ti-44.5Al-6.25Nb-0.8Mo-0.1B) γ-TiAl or the orthorhombic Ti-22Al-25Nb. Generally, PBF-LB/M of titanium aluminides requires high preheating temperatures of the build space in order to tackle the high brittle-to-ductile transition temperatures of these intermetallics. In this work we developed a high-temperature laser powder bed fusion process for γ-TiAl Ti48-2-2 (Ti-48Al-2Cr-2Nb) and applied it to manufacturing and testing of light-weight compressor wheels in order to study the advantages and draw-backs of the high-temperature PBF-LB/M approach for such applications. Challenges related to the required high pre-heat temperatures, the microstructure formation, phase compositions and phase conversions under different processing and post-processing conditions where studied with a variety of methods including synchrotron in-situ high energy x-ray diffraction and the desired material subsequently adjusted. Chemical problems such as minimizing Al evaporation as well as oxygen pick-up due to traces of residual gases or humidity were investigated and addressed. A manufacturing strategy for thin-walled structures and overhanging compressor blades without supports causing high post-processing efforts or deteriorating surface qualities was developed and put to use for manufacturing load- and weight-optimized compressor wheels. The build components were characterized using geometric inspection and computer-tomographic analysis. The achieved material properties and the part design and were successfully validated in spin-tests, highlighting the technical feasibility of using high-temperature PBF-LB/M for such demanding applications.